Field of the Invention
The invention relates to integrated enhanced solvent deasphalting and delayed coking processes for production of liquid and gas coking unit products, high quality petroleum green coke, and asphalt.
Description of Related Art
Crude oils contain heteroatomic molecules, including polyaromatic molecules, with heteroatomic constituents such as sulfur, nitrogen, nickel, vanadium and others in quantities that can adversely affect the refinery processing of the crude oil fractions. Light crude oils or condensates have sulfur concentrations as low as 0.01 percent by weight (W %). In contrast, heavy crude oils and heavy petroleum fractions have sulfur concentrations as high as 5-6 W %. Similarly, the nitrogen content of crude oils can be in the range of 0.001-1.0 W %. These impurities must be removed during refining to meet established environmental regulations for the final products (for instance, gasoline, diesel, fuel oil), or for the intermediate refining streams that are to be processed for further upgrading, such as isomerization reforming. Contaminants such as nitrogen, sulfur and heavy metals are known to deactivate or poison catalysts.
In a typical refinery, crude oil is first fractionated in the atmospheric distillation column to separate sour gas including methane, ethane, propane, butanes and hydrogen sulfide, naphtha (36-180° C.), kerosene (180-240° C.), gas oil (240-370° C.) and atmospheric residue, which are the hydrocarbon fractions boiling above 370° C. The atmospheric residue from the atmospheric distillation column is either used as fuel oil or sent to a vacuum distillation unit, depending upon the configuration of the refinery. Principal products from the vacuum distillation are vacuum gas oil, comprising hydrocarbons boiling in the range 370-520° C., and vacuum residue, comprising hydrocarbons boiling above 520° C.
Naphtha, kerosene and gas oil streams derived from crude oils or other natural sources, such as shale oils, bitumens and tar sands, are treated to remove the contaminants, such as sulfur, that exceed the specification set for the end product(s). Hydrotreating of these individual fractions is the most common refining technology used to remove these contaminants. Vacuum gas oil is processed in a hydrocracking unit to produce naphtha and diesel, or in a fluid catalytic cracking (FCC) unit to produce mainly gasoline, light cycle oil (LCO) and heavy cycle oil (HCO) as by-products, the former being used as a blending component in either the diesel pool or in fuel oil, the latter being sent directly to the fuel oil pool.
Heavier fractions from the atmospheric and vacuum distillation units can contain asphaltenes. Asphaltenes are solid in nature and comprise polynuclear aromatics, smaller aromatics and resin molecules. The chemical structures of asphaltenes are complex and include polynuclear hydrocarbons having molecular weights up to 20,000 joined by alkyl chains. Asphaltenes also include nitrogen, sulfur, oxygen and metals such as nickel and vanadium. They are present in crude oils and heavy fractions in varying quantities. Asphaltenes exist in small quantities in light crude oils, or not at all in all condensates or lighter fractions. However, they are present in relatively large quantities in heavy crude oils and petroleum fractions. Asphaltenes have been defined as the component of a heavy crude oil fraction that is precipitated by addition of a low-boiling paraffin solvent, or paraffin naphtha, such as normal pentane, and is soluble in carbon disulfide and benzene. In certain methods their concentrations are defined as the amount of asphaltenes precipitated by addition of an n-paraffin solvent to the feedstock, for instance, as prescribed in the Institute of Petroleum Method IP-143. The heavy fraction can contain asphaltenes when it is derived from carbonaceous sources such as petroleum, coal or oil shale. There is a close relationship between asphaltenes, resins and high molecular weight polycyclic hydrocarbons. Asphaltenes are hypothesized to be formed by the oxidation of natural resins. The hydrogenation of asphaltic compounds containing resins and asphaltenes produces heavy hydrocarbon oils, that is, resins and asphaltenes are hydrogenated into polycyclic aromatic or hydroaromatic hydrocarbons. They differ from polycyclic aromatic hydrocarbons by the presence of oxygen and sulfur in varied amounts.
Upon heating above about 300-400° C., asphaltenes generally do not melt but rather decompose, forming carbon and volatile products. They react with sulfuric acid to form sulfonic acids, as might be expected on the basis of the polyaromatic structure of these components. Flocs and aggregates of asphaltenes will result from the addition of non-polar solvents, for instance, paraffinic solvents, to crude oil and other heavy hydrocarbon oil feedstocks.
Therefore, it is clear that significant measures must be taken during processing of crude oils and heavy fractions to deal with asphaltenes. Failure to do so interferes with subsequent refining operations.
There are several processing options for the heavy fractions such as vacuum residue, including hydroprocessing, coking, visbreaking, gasification and solvent deasphalting. In the solvent deasphalting process, the asphalt fraction, for instance, having 6-8 W % hydrogen, is separated from the vacuum residue by contact with a paraffinic solvent (for instance, C3-C7) at or below the solvents' critical temperatures and pressures. The deasphalted oil, for instance, having 9-11 W % hydrogen, is characterized as a heavy hydrocarbon fraction that is free of asphaltenes and is typically passed to other conversion units such as a hydrocracking unit or a fluid catalytic cracking unit to produce lighter, more valuable fractions.
Deasphalted oil contains a high concentration of contaminants such as sulfur, nitrogen and carbon residue which is an indicator of the coke forming properties of heavy hydrocarbons and defined as micro-carbon residue (MCR), Conradson carbon residue (CCR) or Ramsbottom carbon residue (RCR). MCR, RCR, CCR are determined by ASTM Methods D-4530, D-524 and D-189, respectively. In these tests, the residue remaining after a specified period of evaporation and pyrolysis is expressed as a percentage of the original sample. For example, deasphalted oil obtained from vacuum residue of an Arabian crude oil contains 4.4 W % of sulfur, 2,700 ppmw of nitrogen, and 11 W % of MCR. In another example, a deasphalted oil of Far East origin contains 0.14 W % sulfur, 2,500 ppmw of nitrogen, and 5.5 W % of CCR. These high levels of contaminants, and particularly nitrogen, in the deasphalted oil limit conversion in hydrocracking or FCC units. The adverse effects of nitrogen and micro-carbon residue in FCC operations have been reported to be as follows: 0.4-0.6 W % higher coke yield, 4-6 V % less gasoline yield and 5-8 V % less conversion per 1000 ppmw of nitrogen. (See Sok Yui et al., Oil and Gas Journal, Jan. 19, 1998.) Similarly, coke yield is 0.33-0.6 W % more for each one W % of MCR in the feedstock. In hydrocracking operations, the catalyst deactivation is a function of the feedstock nitrogen and MCR content. The catalyst deactivation is about 3-5° C. per 1000 ppmw of nitrogen and 2-4° C. for each one W % of MCR.
It has been established that organic nitrogen is the most detrimental catalyst poison present in the hydrocarbon streams from the sources identified above. Organic nitrogen compounds poison the active catalytic sites resulting in catalyst deactivation, which in turn reduces catalyst cycle process length, catalyst lifetime, product yields, and product quality, and also increases the severity of operating conditions and the associated cost of plant construction and operations. Removing nitrogen, sulfur, metals and other contaminants that poison catalysts will improve refining operations and will have the advantage of permitting refiners to process more and/or heavier feedstocks.
In coking processes, heavy feeds are thermally decomposed to produce coke, gas and liquid product streams of varying boiling ranges. Coke is generally treated as a low value by-product. It is removed from the units and can be recovered for various uses depending on its quality.
The use of heavy crude oils having high metals and sulfur content as an initial feed is of interest due to its lower market value. Traditional coking processes using these feeds produce coke which has substantial sulfur and metal content. The goal of minimizing air pollution is a further incentive for treating residuum in a coking unit since the gases and liquids produced contain sulfur in a form that can be relatively easily removed.
While individual and discrete solvent deasphalting and coking operations are well developed and suitable for their intended purposes, there remains a need for improved processes using heavy feeds having asphaltenes, N, S and metal contaminants.